In order to achieve access independence and to maintain a smooth interoperation with wired terminals across the internet, the IMS as specified e.g. in the 3GPP specification TS 23.228 has been developed to be conformant to IETF (Internet Engineering Task Force) “Internet Standards”. The IP multimedia core network (IM CN) subsystem enables network operators of mobile or cellular networks to offer their subscribers multimedia services based on and built upon Internet applications, services and protocols. The intention is to develop such services by mobile network operators and other 3rd party suppliers including those in the Internet space using the mechanisms provided by the Internet and the IM CN subsystem. The IMS thus enables conversions of, and access to, voice, video, messaging, data and web-based technologies for wireless users, and combines the growth of the Internet with the growth in mobile communications.
FIG. 1 shows an architecture of an IMS network according to the above 3GPP (3rd Generation Partnership Project) specification. The architecture is based on the principle that the service control for home subscribed services for a roaming subscriber is in the home network HN, e.g. a Serving Call State Control Function (S-CSCF) is located in the home network HN. In FIG. 1, a current or old S-CSC-Fo 10 and a future or new S-CSCFn 12 are shown, between which a terminal device or user equipment (UE) 40 is to be transferred due to changed required capabilities resulting from a change in the subscriber profile of the UE 40.
In general, an S-CSCF performs the session control service for the served UEs. It maintains a session state as needed by the network operator for support of the services. Within an operator's network, different S-CSCFs may have different functionalities. The functions performed by the S-CSCF during a respective session are e.g. registration, session flow management, charging and resource utilization management. When a subscriber roams to a visited network VN, the visited network VN supports a Proxy-CSCF (P-CSCF) 30 which enables the session control to be passed to the respective S-CSCF located at the home network HN and providing the service control. Furthermore, an Interrogating-CSCF (I-CSCF) 50 is provided in the home network HN as a contact point within the operator's network for all connections destined to a subscriber of that network operator, or a roaming subscriber currently located within that network operator's service area. There may be multiple I-CSCFs within an operator's network. The functions performed by the I-CSCF 50 include assigning an S-CSCF to a user performing a registration procedure, routing a request received from another network towards the S-CSCF, maintaining the address of an S-CSCF from a subscriber database, e.g. a Home Subscriber Server (HSS) 20 as shown in FIG. 1, and/or forwarding requests or responses to the S-CSCF determined based on the address of change from the HSS 20.
The P-CSCF 30 is the first contact point within the IMS. Its address is discovered by the UE 40 following a PDP (Packet Data Protocol) contact activation. The P-CSCF 30 behaves like a proxy, i.e. it accepts requests and services them internally or forwards them on, possibly after translation. The P-CSCF 30 may also behave as a User Agent, i.e. in abnormal conditions it may terminate and independently generate transactions. The functions performed by the P-CSCF 30 are forwarding register requests received from the UE 40 to an I-CSCF, e.g. the I-CSCF 50, determined using the home domain name as provided by the UE 40, and forwarding requests or responses to the UE 40.
Further details regarding the functions of the different CSCF elements shown in FIG. 1 can be gathered from the above mentioned 3GPP-specification.
According to the conventional network architecture in the above mentioned 3GPP Release 5 specification, the HSS 20 is not aware of the kind of capabilities a specific S-CSCF has in the network. On the contrary, the HSS 20 knows what kind of capabilities an S-CSCF needs to support a specific subscriber. This information is stored in a subscriber profile of the specific subscriber. During an initial registration process of UE 40, the HSS 20 sends the required S-CSCF capabilities to the I-CSCF 50 and the actual selection of the S-CSCF is done by the I-CSCF 50. The selection at the I-CSCF 50 is performed on the basis of an information indicating the required capabilities and received from the HSS 20.
However, when there is a need for updating the subscriber profile e.g. in the S-CSCFo 10 currently serving the UE 40, the HSS 20 cannot know whether the selected S-CSCFo 10 is still capable of adequately serving the subscriber of the UE 40. It may be possible that new capabilities required according to the new subscriber profile are not supported by the S-CSCFo 10. Another possibility is that the service provider has removed some service from the subscriber profile and thus has prevented the usage of this service or service part.
If the capability of the S-CSCFo 10 does not meet with the updated subscriber profile, the subscriber is not able to use all subscribed services, or may received services which he or she is no longer willing to have. Furthermore, the subscriber may be charged for services which he or she has been cancelled. Moreover, if the network operator has denied services, the subscriber may still be able to use these services which he or she is no longer authorized to use.